In operation of a high intensity metal halide discharge lamp, visible radiation is emitted by the metal portion of the metal halide fill at relatively high pressure upon excitation typically caused by passage of current therethrough. One class of high intensity metal halide lamps comprises electrodeless lamps which generate an arc discharge by establishing a solenoidal electric field in the high-pressure gaseous lamp fill comprising the combination of one or more metal halides and an inert buffer gas. In particular, the lamp fill, or discharge plasma, is excited by radio frequency (RF) current in an excitation coil surrounding an arc tube which contains the fill. The arc tube and excitation coil assembly acts essentially as a transformer which couples RF energy to the plasma. That is, the excitation coil acts as a primary coil, and the plasma functions as a single-turn secondary. RF current in the excitation coil produces a time-varying magnetic field, in turn creating an electric field in the plasma which closes completely upon itself, i.e., a solenoidal electric field. Current flows as a result of this electric field, producing a toroidal arc discharge in the arc tube.
Typical electrodeless metal halide discharge lamps use metal halides (e.g., including at least one metal iodide) for generating white color lamp emission for general lighting applications. Disadvantageously, however, free iodine formation and devitrification of the arc tube wall occur in electrodeless high intensity metal halide discharge lamps after exposure to the plasma arc discharge. The amount of free iodine in the arc tube increases with time. This accumulating iodine, beyond a certain threshold, causes arc instability and eventual arc extinction.
Accordingly, it is desirable to control the iodine level in electrodeless high intensity metal halide discharge lamps and thereby promote arc stability, while extending lamp life and improving lamp performance.